So far I've brewed 21 Mr. Beer recipes and I can say none of them has come out tasting like sherry or cardboard. I follow their standard brewing guidelines by first pouring 1 gallon of cool water into the 2 gallon fermenter keg. I then boil 3 cups of water, remove the water from the heat source before adding in my extracts, hops and adjuncts as called for by the recipes. I pour this mixture (about 2 to 3 quarts) into the fermenter containing the 1 gallon of cool water before topping the keg off to the full 8.5 capacity.
Without using a thermometer I can tell that the wort in the fermenter at this point of the process is at the right temperature for pitching yeast. The wort feels just a little warm to the touch and this process has produced 21 batches of the best tasting beers I've ever had.